Generally, in a cloth feeding mechanism of a sewing machine, the cloth feeding operation stroke is made adjustable.
Various kinds of contrivances are, presently, employed to adjust the cloth feeding operation stroke. For example, there is a system wherein an eccentric cam is exchanged. Also, there is a system wherein the distance of fitting a main shaft, eccentrically driving a horizontally feeding rod to the core, is adjusted.
In the system wherein the eccentric cam is exchanged, there is encountered the troublesome operation of replacing the cam. Also, a link mechanism allowing the replacement of the cam must be selected. This is very disadvantageous to the operation and design of the sewing machine.
In the system where the position of fitting the main shaft eccentrically driving the horizontally feeding rod to the core is adjusted, a very high precision is required for the manufacture of parts. Also, the link mechanism is so large as to be disadvantageous and inconvenient to the design of the sewing machine.
Furthermore, and with respect to an overlock sewing machine, such machine may or may not be provided with a main cloth feed and auxiliary cloth feed, as cloth feeding devices.
In the sewing machine provided with the main and auxiliary cloth feeding devices, it is required that the differential ratio be large enough; be freely adjustable between normal and reverse, and be simply adjusted by a means provided outside the machine frame. However, in a basic driving method already adopted in a differential cloth feeding device, power is transmitted from one cam to the main and auxiliary feeding arms through a link in which the distance between the fulcrum and operating point can be adjusted. Alternatively, two cams are used so as to respectively drive the main and auxiliary cloth feeding arms and are themselves exchanged to adjust the differential ratio.
In the former of such typical conventional methods, the link must be long so that the stroke, for adjusting the distance between the fulcrum and operating point, when transmitting the power to the main and auxiliary cloth feeding arms, may be along enough. This means that the operating mechanism part of the cloth feeding arm must be large. This inhibits the manufacture of small sewing machines. Moreover, only the operating stroke of the auxiliary cloth feed can be adjusted by operating a lever from outside the machine frame. Yet, the pivotal point of the link must be varied by inserting a spanner or the like into the narrow complicated cloth feeding link mechanism below the bed in order to adjust the main cloth feed. It has been very difficult, however, to make the differential ratio of the main and auxiliary cloth feeds large enough and still be able to obtain a reverse differential ratio, as used in the case of an extended sewing. In fact, in the case of such extended sewing, the maximum ratio has been about 1 : 1.4.
On the other hand, in the case of the latter method wherein the cams are exchanged, the work of exchanging the cams is still troublesome and is not desirable to the work efficiency.